130 PREPARATION AND MOUNTING 
another piece is to be attached should be laid thin strips of 
the glue; both pieces must then be heated upon a small 
brass table, with the aid of the spirit-lamp, until the strips 
become melted; the small piece is then to be taken up and 
placed upon the spot to which it is to be attached, and so 
on until the cell is completed. . It will be found necessary to 
spread the glue over the surface required with a needle or 
some other instrument, so that an unbroken line may be 
presented to the wall of the cell, and no bubbles formed. 
Too great a heat will burn the marine-glue, and render it 
brittle ; care must be therefore taken to avoid this. 
When shallow cells are required, those which are made 
by grinding a concavity in the middle of an ordinary slide 
will be found very convenient. The concavities are cut both 
circular and oblong; and the surface being flat, the cover is 
easily fastened upon it. These are now cheap, and are very 
safe as to leakage. It is a very great improvement, where 
it can be done, to turn a shallow ring outside the concavity 
of the slide, but close to it. This prevents the cement with 
which the cover is fastened from running in. 
Circular cells with a flat bottom used to be made by 
drilling a hole through glass of the required thickness, and 
fixing this upon an ordinary slide with marine-glue; but 
' the danger of breakage and the labour were so great that 
this method is seldom used now, and, indeed, the rings about 
to be mentioned do away with all necessity for it. 
Guass Rines.—Where any depth is required, no method 
of making a cell for liquids is so convenient as the use of 
glass rings, which are now easily and cheaply procurable. 
They are made of almost every size and depth, and, except 
in very extraordinary cases, the necessity for building cells 
is completely obviated. These rings have both edges left — 
rough, and consequently adhere very well to the slide, this 
adherence being generally accomplished by the aid of 
marine-glue, as before noticed with the glass cells. Gold- 
size has been occasionally used for this purpose; and the 
adherence, even with liquid in the cell, I have always found 
